Myoclonus is a brief, shocklike contraction of a muscle or group of muscles. Diagnosis is clinical and sometimes confirmed by electromyographic testing. Treatment includes correction of reversible causes and, when necessary, oral drugs to relieve symptoms.

Physiologic myoclonus may occur when a person is falling asleep and during early sleep phases (called hypnic myoclonus). Hypnic myoclonus can be focal, multifocal, segmental, or generalized and may resemble a startle reaction. Another type of physiologic myoclonus is hiccuping (diaphragmatic myoclonus).

Pathologic myoclonus can result from various disorders and drugs (see Table: Causes of Myoclonus). The most common causes are

Hypoxia

Drug toxicity

Metabolic disturbances

Other causes include degenerative disorders affecting the basal ganglia and some dementias.

Classification of myoclonus

Myoclonus may be classified based on the site of origin as follows:

Cortical: This type of myoclonus is associated with cerebral cortex damage or epilepsy. Photic visual stimuli or touching may trigger myoclonic jerks, which may cause abnormalities on an EEG (eg, focal or generalized spike-and-wave or polyspike-and-wave epileptiform discharges, giant somatosensory evoked potentials). The myoclonic jerks may be less evident at rest but aggravated during motor action.

Subcortical: This type of myoclonus is associated with disorders that affect the basal ganglia It is similar to cortical myoclonus. However, there are no EEG abnormalities or giant somatosensory evoked potentials, and photic visual stimuli are not a trigger.

Reticular: This type of myoclonus is thought to originate in the brain stem. It is similar to hyperexplexia (hypertonia and an exaggerated startle reaction). But in reticular myoclonus, unlike in hyperexplexia, myoclonus frequently occurs spontaneously and is more likely to be triggered by touching the distal limbs rather than the head, face and/or upper chest. Reticular myoclonus can also be triggered by movement. Myoclonic jerks usually affect the whole body, with muscles on both sides of the body affected simultaneously.

Peripheral: This type of myoclonus results from damage to peripheral nerves, nerve roots, or plexuses. It is characterized by rhythmic or semirhythmic jerks. Hemifacial spasm is an example of peripheral myoclonus.

Classifying myoclonus based on site of origin is thought to be the most helpful when choosing the most effective treatment.

Symptoms and Signs

Myoclonus that occurs when patients are suddenly startled (startle myoclonus) may be an early symptom of Creutzfeldt-Jacob disease.

Myoclonus due to severe closed head trauma or hypoxic-ischemic brain damage may worsen with purposeful movements (action myoclonus) or may occur spontaneously when movement is limited because of injury.

Myoclonus due to metabolic disturbances may be multifocal, asymmetric, and stimulus-induced; it usually involves facial or proximal limb muscles. If the disturbance persists, generalized myoclonic jerks and, ultimately, seizures may occur.

Diagnosis

Clinical evaluation

Diagnosis of myoclonus is clinical. Testing is done based on clinically suspected causes.

Treatment

Correcting the metabolic disturbance or other cause if possible

Stopping or reducing the dose of the causative drug

Drug therapy to relieve symptoms

Treatment of myoclonus begins with correction of underlying metabolic disturbances or other causes if correctable. If a drug is the cause, the drug is stopped, or the dose is reduced.

For symptom relief, clonazepam 0.5 to 2 mg po tid is often effective. Valproate 250 to 500 mg po bid or levetiracetam 250 to 500 mg po once/day to bid may be effective; rarely, other anticonvulsants help. Doses of clonazepam or valproate may need to be lower in the elderly.

Site of origin for myoclonus can help guide treatment. For example, valproate, levetiracetam, and piracetam tend to be effective in cortical myoclonus but ineffective in other types of myoclonus. Clonazepam may be effective in all types of myoclonus. In some cases, a combination of drugs is necessary.

Many types of myoclonus respond to the serotonin precursor 5-hydroxytryptophan (initially, 25 mg po qid, increased to 150 to 250 mg po qid), which must be used with the oral decarboxylase inhibitor carbidopa (50 mg every morning and 25 mg at noon or 50 mg every evening and 25 mg at bedtime).

Key Points

Myoclonus is a brief, shocklike muscle contraction that can vary in severity and distribution.

Myoclonus can be physiologic (eg, hiccuping, sleep-related muscle contractions) or secondary to various brain disorders, systemic disorders, or drugs.

If a metabolic disturbance is the cause, correct it, and when necessary, give drugs (eg, clonazepam, valproate, levetiracetam) to relieve symptoms.

Last full review/revision February 2017 by Hector A. Gonzalez-Usigli, MD

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